With semiconductor architectures becoming more and more small and complex, 3D structured NAND has been highly desirable as memory cells are stacked on top of each other to increase capacity through higher density, lower cost per gigabyte, and offer the reliability, speed and performance expected of solid-state memory. In the field of 3D NAND fabrication, generally, photoresists are applied over a stack of layers of various materials to be patterned in subsequent processing steps. To take advantage of the spatial resolution of the photoresists, it is necessary to use an anti-reflective coating (ARC) layer underlying the photoresist, to suppress reflection off other layers in the stack during photoresist exposure. The ARC layer typically refers to one or multiple layers of ARC, for example, a bottom anti-reflective coating (BARC) layer formed of organic compositions and a dielectric anti-reflective coating (DARC) layer formed of inorganic compositions. Recently there has been increased interest in the use of silicon oxynitride (SiON) as an inorganic DARC, due to its ability to function well in combination with deep ultraviolet (UV) photoresists.
In 3D NAND applications, the thickness of the SiON layer below PR mask layer for lithography purpose is increased due to more and more layers of various materials to be patterned added below the SiON layer. The PR mask layer typically is a spin-on material consisting of C, H and O element, which is easily damaged by plasma. Traditional dry etch gases, such as CF4, CHF3, C4F8 or C4F6 has limited SiON/PR selectivity. These gases tend to etch isotopically, and create overhanging/damage on the PR mask layer; hence the damages on PR or the structure changes on the PR will affect the subsequent etching steps. Thus achieving high selectivity of SiON/PR with minimum PR deformation is challenging and has attracted significant attention from industry.
Attempts have been performed to inhibit damage of the PR mask layer during etching processes under plasma etching conditions, that is, to improve the selectivity of the DARC layer to photoresist layer.
Hydrofluorocarbons or fluorocarbons have been used to etch the DARC layer and the dielectric layer over which a photoresist layer is deposited. For example, U.S. Pat. No. 6,495,469 to Yang et al. disclose etching a DARC layer employing a CH3F, CH2F2, or CHF3 with O2/N2/Ar improves a selectivity of the DARC layer and the dielectric layer to photoresist layer from about 0.87 to 2.45.
Furthermore, stacks of silicon oxide and silicon nitride (SiO/SiN or ON) and silicon oxide and polysilicon (SiO/p-Si or OP) are important compositions of tunnel and charge trapping in NAND type flash memory. Etching of stacks of multiple SiO/SiN or SiO/p-Si layers is critical in 3D NAND applications. The challenge of etching 3D NAND is that how to etch oxide and nitride layers or oxide and polysilicon (p-Si) layers with a similar etch rate as high as possible. In addition, the etched structure should have a straight vertical profile without bowing and low line etch roughness (LER).
Traditional etch gases for etching SiO/SiN or SiO/p-Si layers include cC4H8, C4F6, CF4, CH2F2, CH3F, and/or CHF3. It is known that selectivity and polymer deposition rate increase as the ratio of C:F increases (i.e., C4F6>C4F8>CF4). Traditional etch chemistries may not be able to provide a feature, such as a hole or trench, having an aspect ratio higher that 20:1, which is necessary in the newer applications (e.g., 3D NAND), at least due to insufficient etch resistant polymer deposition on sidewalls during the plasma etching processes. The sidewall —CxFy— polymers, wherein x ranges from 0.01 to 1 and y ranges from 0.01 to 4, may be susceptible to etching. As a result, the etched patterns may not be vertical and the etch structure may show bowing, change in dimensions, pattern collapse and/or increased roughness.
Nitrogen containing compounds have been used as etching gases. For example, U.S. Pat. Nos. 6,569,774 and 7,153,779 to Trapp disclose plasma etch process for forming a high aspect ratio contact opening through a silicon oxide layer. At least one etch gas is used that includes one or more nitrogen-comprising gases to deposit a polymeric surface material during the etching for maintaining a masking layer over the silicon oxide layer. A list of hydrofluorocarbon and fluorocarbon containing —NH2 chemistries are disclosed, but no structural formulae, CAS numbers, or isomer information are provided. U.S. Pat. No. 9,659,788 to Surla et al. discloses nitrogen-containing using —NH2 containing etching gas for plasma etching silicon-containing films, in which 1,1,1,3,3,3-Hexafluoroisopropylamine (C3H3F6N) is disclosed to offer sidewall protection and good selectivity to p-Si and a-C but lose selectivity to SiN film even without any oxygen addition.
Up to now, using nitrogen-containing hydrofluorocarbons to etch both DARC layer and the stacks of silicon-containing layers has not been found. There is, therefore, a need to develop new etch gas compositions for use in patterning a stack of layers of various materials in plasma etching applications, which may provide high selectivity of the DARC layer versus the PR mask layer and the silicon-containing layers versus a-C layer and maintain high aspect ratio for a wide range of process conditions.